I am not sure of a formula, but if memory serves me if velocities reach or exceed 550 to 600 fpm it will cause condensate to be carried off of the coil.

 

Work your cfm formula backwards if you know cfm already and you will know duct velocity. The only way to know true face velocity is to use an anemometer and measure it unless you also have the figures on exactly how much of the coil face area the fins take up.

 

I thought you could use pressure drop and coil dimensions
face velocity= square root of p.d. /face dimensions x 4005 ??????
I can't remember !!!
coil=4ft x 3.5ft p.d. =.4"wc

 

I believe what you're talking about is finding the current conditions based on having a known point to work from, but that's the trick - you have to have a known point. That equation is: pressure drop 1 / pressure drop 2 = (velocity 2 / velocity 1) squared. Your curve for a particular piece of gear will give you either a true operating curve or enough preplotted points to get you where you're going when using that formula. It may or may not give you max face velocities, but you can get these from the mfr. These will change from coil to coil, but generally stay in a range. If you're experiencing carryover, your best bet by far is to measure with a meter right at the coil face. That way you not only get average flow, but you see the specific areas of increased flow. Your average may not be high, but you may be getting areas of increased velocity that will create a problem.

There very well could be something else that you're looking for, but after searching, I couldn't find a specific formula for it. I'll keep looking. Maybe in the meantime, one of the T&B guys will drop by share it.

 

Thanks Klove, as always your responses are very informative and professional

 

Are you thinking of that old thumb rule of 500 FPM Max face velocity? so Sq Ft of coil times 500 should be your CFMs Less should be better like 400 FPM

 

CFM = velocity x surface (in square feet)

Therefore

Velocity = CFM devide Surface (in square feet)

 

So. I could turn the heat on and get delta T and use-btu output x 1.08 /delta T to obtain CFM, then use CFM /area of coil in sqft. to obtain face velocity in FPM. Under 350 should be good ?

 

I hope this is what you are looking for?

 

NOPE, that would not give you an answer because the CFMs dry vs wet are going to be different airflows and even if you tried that you would need to know exact GPM with that temp diff and even more you still need take into consideration this is not dry air.
The only answer is to use a pitot tube and traverse the duct and get the average velocity and DO the MATH

 

Found this thread while web searching, as I had a similar problem myself. It doesn't help that most of my reference materials are at home, while I needed an answer right now at the office.

When using a rotating vane anemometer (in the absence of a good duct traverse location), what I found is:

The AABC folks have the formula CFM = (area in ft2) x (velocity in fpm x correction factor).

The correction factor is a function of coil fin spacing, tube spacing, tube diameter etc etc. The AABC book on balancing has a rather nice formula to work through.

I seem to recall there was a rule of thumb correction factor but can't find the reference and obviously from one coil to another it could vary widely (from memory I seem to recall the rule of thumb is 0.7 but still looking).

Here is a sample of when ASHRAE looked into it from the research point of view: http://rp.ashrae.biz/page/RP451_(2_phases).pdf

 

I have some comments on this from testing FV's in a lab setting but I'll have to wait till I get nqck to my old reference books and notes.

 

I moved this to Tech-to-tech commercial.

 

how to calculate face velocity on coil

 

One less likely possibility; How is the superheat? I had a package unit years ago fling water past the pan because the coil was not being feed evenly. It was dry in sections and water seemed to blow off at the edge points of dryness. The speed of the blower had not been changed and the coil was cleaned as the first solution try. Fixed superheat and I was surprised the problem went away.

 

where could i get editable exel calculator for checking all AHU formulas
example CFM, FPM, etc,...

 

guys, just a quick question. i'm working on improving our factory's air handling system, and examining the psychro charts of the ahu shows that moisture is gained when the air passes through the reheat coils. our air handling system uses a dx cooling and reheat coil system. currently, the evaporator coil and the reheat coil are approx. a foot and a half apart from each other, which is the area where the condensate drains off to. im afraid the the moisture gain during reheat is due to the condensate carry over. are there any solutions to my problem which would not necessitate a redesign on that part of the ahu?